a) Field of the Invention
The invention is directed to an arrangement for the generation of intensive short-wavelength radiation based on a plasma, wherein high-energy excitation radiation is directed to a target flow in the vacuum chamber and, by means of a defined pulse energy, completely transforms portions of the target flow into a dense, hot plasma which emits particularly short-wavelength radiation in the extreme ultraviolet (EUV) range, i.e., in the wavelength region of 1 nm to 20 nm.
b) Description of the Related Art
The invention is used as a light source of short-wavelength radiation, preferably for EUV lithography in the production of integrated circuits. However, it can also be used for incoherent light sources in other spectral regions from the soft x-ray region to the infrared spectral region.
In order to produce increasingly faster integrated circuits, it is necessary for the width of the individual structure on the chip to be increasingly smaller. Since the resolution in optical methods (optical lithography) is proportional to the wavelength of the light that is used, development is toward increasingly smaller wavelengths. An area with very good prospects for the future is EUV lithography (wavelength around 13.5 nm).
In the interest of economy, a determined throughput of wafers must be ensured, which necessitates a light source having a high minimum output at a defined efficiency of the imaging optics. At the present time, there are no light sources in the wavelength region around 13.5 nm that would be capable of providing the required outputs. Also, the selection of light sources which could potentially be capable of this is very limited.
Based on the present state of knowledge, laser-produced plasmas, discharge plasmas and synchrotrons are the most promising radiation sources for EUV lithography. Sources based on a plasma have the advantage that they can be incorporated relatively easily in existing production processes.
“Mass-limited” targets were developed in order to limit unwanted particle emission in laser-produced plasmas which could sharply reduce the life of the plasma facing optics in particular. These mass-limited targets substantially reduce the amount of debris produced. In this connection, mass-limited means that the available target material is completely transformed into plasma by interaction with the energy beam. Since the amount of material available for generating radiation is therefore limited, the amount of energy in the beam pulse is exactly that amount needed for optimal conversion of, e.g., laser photons into EUV photons. Accordingly, at a given pulse repetition rate of the energy beam, the average output that can be coupled in is fixed and, at a determined conversion efficiency, so also is the maximum EUV output that can be generated. The maximum pulse repetition rate of the energy beam is given in that the target is disturbed through the plasma generation, and a minimum time interval between the individual laser pulses which depends on the transport speed of the target flow is therefore necessary.
Target concepts that have already been suggested include:                a continuous material jet (target jet) comprising, e.g., condensed xenon (e.g., according to WO 97/40650 A1);        a dense droplet mist comprising microscopically small droplets (e.g., WO 01/30122 A1);        cluster targets (e.g., U.S. Pat. No. 5,577,092);        macroscopic droplets (e.g., EP 0 186 491 B1); and        ice crystals through the use of a spray (U.S. Pat. No. 6,324,256).        
In all of the known target concepts, the amount of material available for an excitation pulse is small, so that the maximum energy of the individual pulse is limited. The transport speed of the target material and the diameter of the target jet can also not be increased to an unlimited extent for physical reasons (hydrodynamics), so that the pulse repetition rate of the energy beam is limited also. Since the average output is given by the product of individual pulse energy and repetition rate of the excitation signal, there is an upper limit for the EUV output that can be generated. Accordingly, with conventional targets it is not possible to reach the high average outputs in the EUV spectral region that are required by the semiconductor industry.